A battery is one of the most important energy storage devices in high-power or high-capacity energy storage applications such as electric vehicles, and in renewable energy generation systems such as distributed wind and photo-voltaic (PV) power generation. A battery may be used to smooth intermittent power generated from distributed energy resources, and to make generated power more distributable. In conventional high-power battery packs, battery cells are connected in series to increase the battery output voltage, making it is easier and more efficient to interface with a high voltage system DC bus.
Major challenges for conventional batteries include cost, cell capacity and life, power density, energy density, and safety. Variations in battery cell capacity and internal resistance are inevitable both in manufacturing process and during operation. This imposes unnecessary costs in terms of money and efficiency. For example, during the charging process, battery cells with lower capacity are charged more quickly than those with larger capacity. Therefore, even though the lower capacity cells may reach their maximum voltage cell voltage, they must continue to be charged so that the higher capacity cells can reach their maximum voltage. This further charging causes the weaker cells to overcharge, and causes irreversible damage to the electrodes of the weaker cells, which results in a decreased battery capacity. Therefore, weaker battery cells degrade much faster than stronger ones, and the presence of weaker cells reduces the operational capacity and lifetime of the entire battery. A similar outcome occurs as a result of the discharging process. Improving the manufacturing process or controlling ambient conditions for better cell consistency involves significant increases in cost.
There are a few conventional approaches to mitigate these issues and improve battery pack performance. One such approach is a battery management system (BMS), which acquires and processes operational cell characteristic data. Typical monitored characteristics include cell voltage, cell temperature, and operating current. Typical functionalities include over-voltage and under-voltage protection, over-temperature protection, and over-current or short circuit protection. Advanced BMS also estimates the state of charge (SOC) and the state of health (SOH) of a battery pack, and relays the states to an external controller. A major issue for this approach is that, in order to minimize cell damage and extend battery life, only part of the rated battery pack capacity can be used. For example, in today's on-board lithium ion battery pack of a typical plug-in electric vehicle (PHEV), only 50% of the battery pack capacity is usable.
A second approach is cell equalizer technology. Cell equalizer technology avoids over-charging or over-discharging by directing current away from weaker cells when they charge to their maximum capacity, and by directing current to weaker cells when they discharge to their minimum voltage threshold. Early equalizer technology employs dissipative balance circuits such as resistors and transistors in low power applications. Since there is high power loss in a dissipative equalizer, the directed current is usually very small compared with the battery current rating, and they can balance cell voltage only during trickle charging. At the end of the charging process, trickle charging with very small current is employed to further charge stronger cells, and to direct current away from weaker cells. But trickle charging may not be applicable to lithium ion batteries because they are sensitive to over-voltage.
Non-dissipative technologies were proposed for high-power applications. For example: a switched capacitor design or a switched inductor design, a DC-DC converter design, a coupled inductor or high-frequency transformers design, and a relay switch matrix design. The directed current flow can be either unidirectional for charging equalization, or bidirectional for both charging and discharging equalization. There are several issues for non-dissipative cell equalizers such as DC-DC converters. For example, there is trade-off between the cost of an equalizer and the equalization speed. An equalizer with a higher current rating can provide faster equalization speed and balance voltage dynamically with inrush current (e.g., regenerative braking of electric vehicle). But this approach will generate additional cost, volume, and weight of the energy storage system. Most reported equalization times are in the order of magnitude of minutes to hours, which is difficult for the transient voltage balance of high power battery cells. Another issue is the reliability of the battery. When one battery cell fails, the cell equalizer circuits have to provide a bypass branch to isolate the failed cell, to ensure that the battery remains operable. However, it is difficult to directly utilize equalizer circuits to provide bypass branches. In addition, the current rating of bypass branches should be the same as the maximum output current of the battery. Such branches are not cost effective because they are in standby during normal operation. Another issue is with the control algorithm. The equalizer circuits operate by reallocating energy among battery cells, however, allocating energy efficiently and quickly will result in a complicated control system.
Therefore, a need exists for one or more electric power systems and devices that address the issues associated with the conventional approaches for energy power source management.